Low temperature synthesis of ZnO particles using a CO2-driven mechanism under high pressure

Low temperature synthesis of ZnO particles without using reactive materials, solvents and post-treatments is still a serious challenge for both fundamental research and industrial applications. In this research, we report the dry synthesis of ZnO particles only by using Zn(acac)2 and supercritical CO2 (scCO2) at the low temperature of 60 °C. The synthesis was performed using CO2 and N2 from 0.1 to 30.0 MPa for 18 h. As a result, ZnO yields increased with a rise in the CO2 pressure and reached 67% at 30.0 MPa while N2 medium gave low yields below 4.9% regardless of the pressure. Additionally, the detailed characterization results and the phase behavior observations evidentially showed the formation of zinc–CO/CO2–organic complexes in the solid phase of Zn(acac)2 powder under scCO2, resulting in the accelerated formation of ZnO particles. These findings suggest that scCO2 has potential value to drive the formation reaction of zinc–CO/CO2–organic complexes, which allows the low temperature synthesis of ZnO particles under dry conditions without using reactive materials, solvents and post-treatments.


Introduction
Zinc oxide (ZnO) is utilized in cosmetics, 1,2 paints, 3 gas sensors, 4,5 photocatalysts 6,7 and drugs [8][9][10] due to its low toxicity 11 and excellent optical and electrical properties such as a wide band gap of 3.37 eV and a high exciton binding energy of 60 meV. 12Therefore, ZnO particles have been synthesized using various gas and liquid phase methods, such as chemical vapor deposition, 13 spray pyrolysis 14 and plasma, 15 sol-gel, 16 precipitation 17 and hydrothermal methods. 18Although the gas phase methods have the advantage of requiring no post-treatment, they require high temperature conditions, leading to the increased cost and limiting applications. 19Although the liquid phase methods can allow the synthesis of ZnO particles at low temperature, they cause a large amount of liquid waste for the synthesis and washing. 20Therefore, the low temperature synthesis of ZnO particles without a post-treatment is still a serious challenge for the development of a green chemistry approach.
Supercritical CO 2 (scCO 2 ) is known as an environmentally benign medium for the synthesis of inorganic materials due to its nontoxicity, cheapness and recyclability. 21Additionally, scCO 2 has high solubility of metal organic precursor and high diffusivity while the synthesis in scCO 2 is substantially solventless process because scCO 2 is easily removed only by reducing the pressure. 22,23Furthermore, it is revealed in our recent research that scCO 2 can be used as not only synthesis medium but also washing and drying solvents for the particle production. 24These appealing characteristics enable the simple production process without using an organic solvent and a post-treatment.Therefore, scCO 2 has been used as a synthesis eld of various inorganic particles such as metal, [25][26][27] metal oxide [28][29][30][31] and metal sulde. 32In these syntheses, scCO 2 is typically used as a solvent to dissolve precursors for controlling the reaction, which means that scCO 2 has no role to drive the reaction and to directly reduce the reaction temperature.Therefore, low temperature synthesis driven by scCO 2 is a serious challenge for fundamental research and industrial applications.
Similar challenge exists in also the synthesis of ZnO particles from various Zn precursor under scCO 2 .For example, Haldorai et al. report the synthesis of ZnO particles from Zn(NO 3 ) 2 under scCO 2 + ethanol (10 vol%) mixture at 300 °C. 28Vostrikov et al. report the synthesis of ZnO from metallic and bulk zinc material (prepared by casting melted zinc) under scCO 2 or scCO 2 + H 2 O (31.6-82.8mol%) at the temperatures from 330 to 600 °C. 29hereas Chang et al. report the low temperature synthesis of ZnO particles from metallic zinc lm of 80 nm using H 2 O (0.3 vol%) as an oxidant in scCO 2 at 60 °C. 30However, this method requires the preparation of a zinc lm by sputtering, and the reaction only occurs on the nanolm surface, resulting in the slight ZnO quantity.As seen in these previous literatures, some problems of high temperature conditions, much oxidant and low productivity are remained in the ZnO synthesis using various Zn precursors under scCO 2 .Therefore, low temperature and highly productive synthesis should be achieved using a new reaction mechanism driven by scCO 2 without additional oxidant.
In this research, we report the low temperature and highly productive synthesis of ZnO particles from Zn(acac) 2 powder only by using scCO 2 at 60 °C and propose a new reaction mechanism driven by scCO 2 .It is noted that Zn(acac) 2 is powder state and commercially available, which makes it suitable material for highly productive synthesis although the Zn(acac) 2 is typically converted into ZnO at the high temperature of 200 °C under air atmosphere. 33In this work, to demonstrate the potential of scCO 2 , the synthesis was performed under CO 2 and N 2 atmosphere at the pressure from 0.1 to 30.0 MPa.Additionally, the detailed characterizations were applied to the washed and no-washed products to investigate the CO 2 -driven reaction mechanism behind the accelerated formation under scCO 2 .

Synthesis
The high-pressure system, shown in Fig. 1, was used to synthesize ZnO particles under scCO 2 .Aer Zn(acac) 2 of 395 ± 2 mg was loaded into 76 mL reaction vessel (TSC-CO2-008; Taiatsu Glass Corp.), inner air was displaced by owing CO 2 for 1 min under 0.5 MPa.Subsequently, liquid CO 2 was introduced into the vessel using a HPLC pump (PU-4386; JASCO Co., Ltd) until reaching the adequate pressure within 15 min.The inlet and outlet valves were closed and the vessel was sunk in the oil bath at 60 °C, which resulted in the target pressure from 0.1 ± 0.0 to 30.0 ± 0.3 MPa.Aer keeping for 18 h, the vessel was removed from the oil bath and was depressurized at a rate of approximately 0.5 MPa min −1 using a metering valve (1315G2Y; HOKE Inc.).The experiments were also performed at N 2 of 0.1 MPa and 30.0 MPa, where N 2 cylinder was connected to the middle point between the pump and the metering valve.Aer N 2 was introduced into the vessel using the metering valve until reaching the arbitrary pressure, the vessel was sunk in the oil bath, which resulted in the target pressure of 0.1 ± 0.0 and 30.0 ± 1.0 MPa.Other conditions and procedures are the same as the synthesis using scCO 2 .It is noted that the error values of weight and pressure represent the maximum or minimum deviations from the set values for all experiments.
The products were washed using several cycles of sonicationcentrifugation-decantation with 2-methoxyethanol (360 g) and ethanol (20 g).The nal precipitates were dried in a vacuum oven at 22 °C.

Phase observation
The phase behavior was directly observed using a high pressure vessel (volume: 36 mL, Taiatsu Glass Corp.) with sapphire windows.The Zn(acac) 2 of 187 ± 6 mg was enclosed in the vessel, where the weight against the reactor volume was equal to the above synthesis condition.Subsequently, liquid CO 2 was introduced into the vessel until reaching the target pressure of 30.0 ± 0.4 MPa.The inside temperature was kept at 60 °C using a mantle heater for 18 h.Other conditions and procedures are the same as the synthesis of ZnO particles using scCO 2 .

Characterization
The particle yield Y was dened as follows.
where W collect is the weight of products aer washing and drying and W theory is regarded as the weight of ZnO when anhydrous Zn(acac) 2 (263 g mol −1 ) is completely converted to ZnO.The products were analyzed by X-ray diffractometry (XRD) (MiniFlex600-C; Rigaku Corp.) using Cu Ka radiation and by a Fourier transform infrared spectrometer (FT-IR) (FT-IR4100; JASCO Co., Ltd).The products were observed by transmission electron microscopy (TEM) (H-7650; Hitachi Corp.) operated at 100 kV.Thermogravimetric (TG) analysis was performed under N 2 atmosphere using a thermogravimetric analyzer (TGA-50; Shimadzu Corp.).The temperature was increased to 600 °C at a ramp rate of 10 °C min −1 .Thermogravimetric mass analysis (TG-MS) was performed from room temperature to 500 °C at a ramp rate of 10 °C min −1 under He atmosphere using a Thermo plus EVO2 Thermo Mass Photo (Rigaku Corp.).When comparing the relative intensities in each m/z value, the background spectra at 30 °C was subtracted from original mass spectra to eliminate any background noise.In this system, acetylacetone ligands, CO 2 and H 2 O (included in Zn(acac) 2 precursor) are oxygen source.Especially, CO 2 and H 2 O can be possible oxygen donors for the chemical transformation of Zn(acac) 2 to ZnO.As detailly mentioned in next section, the no-washed solid product included CO and H 2 O as major components of thermally released material by TG-MS analysis (Fig. 4a and b), which is evident for the possible following reactions using CO 2 and H 2 O as oxygen donors.

Results and discussion
The CO 2 and H 2 O are reported to be oxygen donors in also the previous literature that describe the ZnO synthesis from metallic zinc under scCO 2 . 29,30However, it should be emphasized that our system requires only the low temperature of 60 °C and slight water [included in Zn(acac) 2 precursor] compared to previous system using high temperature above 300 °C29 and much water of oxidant. 30

Analysis of CO 2 -driven mechanism
To investigate the role of CO 2 behind the accelerated formation, XRD analysis was applied to the no-washed solid product that was synthesized at 30.0 MPa of scCO 2 , as shown in Fig. 3.The XRD pattern showed not only peaks that were assigned to ZnO, Zn(acac) 2 $(H 2 O) 2 and Zn(acac) 2 $H 2 O, but also many new peaks that were not assigned to the possible byproduct of ZnCO 3 , Zn(COO) 2 , Zn(COO) 2 $H 2 O and Zn(OH) 2 at the angle position from 7 to 37°.This result suggests the formation of new zincorganic complexes as intermediate product.To more detailly investigate this complex, TG and TG-MS analyses were applied to the same no-washed product, as shown in Fig. 4a and b.The no-washed product showed the sharp weight loss from 90 to 200 °C for TG analysis while this temperature region resulted in the signicant amount of CO (m/z = 28) and CO 2 (m/z = 44) release from no-washed product for TG-MS analysis.In also the mass spectrum at 116 °C, CO and CO 2 showed strong intensities, where some peaks (m/z = 15, 18, 43, 85, 100) were assigned to Zn(acac) 2 $H 2 O and its fragments. 34These results suggests that scCO 2 medium leads to the formation of zinc-CO/CO 2organic complexes such as Zn(acac) 2 $xCO$yCO 2 , resulting in the low temperature synthesis of ZnO particles.
Furthermore, the formation of zinc-CO/CO 2 -organic complexes may be related to the CO 2 density, namely solvation power of CO 2 , 35 as shown in Fig. 4c.Generally, the solvation power increases with a rise in the CO 2 pressure and sharply increases above the critical pressure (P c = 7.4 MPa), which plausibly accelerates the formation of zinc-CO/CO 2 -organic complexes.Our obtained yields also showed a signicant increase above the critical pressure, as shown in Fig. 2b.These results evidentially show that the formation of zinc-CO/CO 2organic complexes, namely CO 2 -driven reaction mechanism, allows the low temperature synthesis of ZnO particles with high yield, in addition, its effect signicantly increases with a rise in the pressure above the critical point.
The destabilization of Zn precursor by the coordination of CO/CO 2 (corresponding to the formation of zinc-CO/CO 2 complex) is one of the possible mechanisms to accelerate the reaction and subsequent the ZnO formation.Yoda et al. report the synthesis of Cu from Cu(acac) 2 dissolved in scCO 2 at 150-180 °C and the reduction of activation energy by CO 2 solvation. 36ince the coordination is a similar phenomenon to the solvation, the coordination of CO/CO 2 possibly destabilizes the Zn precursor and reduces the activation energy, allowing the low temperature synthesis under scCO 2 .However, the formation of zinc-CO/CO 2 -organic complexes seems to occur in not scCO 2 phase but the solid phase in this work, considering the phase state of enclosed Zn(acac) 2 powder under scCO 2 of 30.0 MPa (Fig. 4d).Interestingly, Zn(acac) 2 powder was not apparently dissolved in scCO 2 and the piled state of enclosed powder did not change before and aer the  reaction of 18 h.This result means that zinc-CO/CO 2 -organic complexes and ZnO particles are formed by not the dissolutionprecipitation mechanism but the solid phase reaction.ScCO 2 has unique characteristics of medium solvation power, high diffusivity and small molecular size, which would allow the rapid penetration of CO 2 into the solid phase, resulting in the formation of zinc-CO/CO 2 -organic complexes and the low temperature synthesis of ZnO particles in the solid phase.Such low temperature synthesis using the formation of zinc-CO/CO 2organic complexes in the solid phase has a large potential value for fundamental research since the typical role of scCO 2 is a solvent to dissolve precursors for the particles production.Furthermore, this system has also a potential value as industrially viable approach with cost-competitiveness since the use of easily available waste heat below 100 °C can minimize the heating cost and solid-phase-like synthesis can allow the mass production due to no requirement of precursor dissolution.

Characterization of ZnO particles
To characterize the morphology and surface structure of ZnO particles, TEM, TG, TG-MS and FT-IR analyses were applied to the washed product synthesized at CO 2 of 30.0 MPa, as shown in Fig. 5.The formation of micron-sized aggregates composed of nano-sized primary particles was validated from the TEM images since the gray contrast difference is clearly observed in a partially magnied area of the particle. 37Herein, TG analysis showed the weight loss of approximately 20% from 135 to 200 °C (Fig. 5b), where the temperature was kept at 100 °C for 20 min to eliminate the physically absorbed water.Whereas TG-MS analysis, especially the relative intensity of the mass spectrum, showed that OH (m/z = 17) and H 2 O (m/z = 18) were the major released components at 165 °C.These results suggest the presence of hydroxyl group on the ZnO surface and crystal water in ZnO bulk structure.It is noted that the CO 2 (m/z = 44) had broad multi-peaks from 200 to 600 °C, which may be due to the desorption/thermolysis of some compounds such as oxalate, acetate and amorphous carbon.In the FT-IR analysis, the bands of Zn(acac) 2 (raw material) were assigned to the stretching vibration of OH (at around 3300 cm −1 ), 38 the stretching vibration of C-H (at 3007 and 2898 cm −1 ), 38 the stretching vibrations of C]C, the bending vibrations of C]CH (at 1590 and 1448 cm −1 ), 39,40 the stretching vibrations of C]O (at 1507 cm −1 ), 41 the stretching vibrations of C]C or C-CH 3 (at 1271 and 933 cm −1 ), 39,42 the rocking of CH 3 (at 1025 cm −1 ) 43 and the out-of-plane bending of C-H bonds (at 776 cm −1 ). 42owever, these bands disappeared or had very low intensities for the product except for broad OH band.Additionally, the product showed the stretching vibration of Zn-O band for the hexagonal wurtzite structure of ZnO (at 532 cm −1 ). 44These results further support the presence of hydroxyl group on the ZnO surface.Moreover, the products showed characteristic peaks at the carboxylate-related region from 1525 to 1402 cm −1 . 43It is reported that acetylacetonate bonded to the surface is easily broken and is transformed into the carboxylate group (-COO − ) due to its unstableness for the formation of ZnO particles. 45Therefore, the characteristic peaks at 1525, 1482 and 1402 cm −1 would reect the -COO − bonded to the ZnO surface of primary particles.Additionally, separated three bands suggest the existence of various coordination modes since the separation of symmetric (1402 cm −1 ) and asymmetric (1525 and 1482 cm −1 ) stretching modes for -COO − typically reects the coordination modes such as unidentate, bridging and bidentate. 46

Proposed mechanism
Our obtained results can be explained by the following mechanism that is also depicted in Fig. 6.Firstly, scCO 2 , with the characteristics of high diffusivity and small molecular size, penetrates into the solid phase of Zn(acac) 2 .Secondly, the contact of Zn(acac) 2 with CO 2 and CO forms zinc-CO/CO 2organic complexes in the solid phase, where CO is formed by CO 2 donates oxygen atom to zinc precursor.Such complex formation allows the low temperature synthesis of ZnO primary particles using CO 2 and H 2 O as oxygen donors.Finally, ZnO primary particles is aggregated while the acetylacetonate on the ZnO surface is transformed into -COO − due to its unstableness.

Conclusions
In this research, to investigate the potential of scCO 2 for the low temperature synthesis of ZnO particles, Zn(acac) 2 of precursor was contacted with CO 2 and N 2 from 0.1 to 30.0 MPa for 18 h at 60 °C.As a result, ZnO yields increased with an increase in the CO 2 pressure and reached 67% at 30.0 MPa, while N 2 medium yielded less than 4.9% regardless of the pressure.Additionally, XRD, TG and TG-MS analyses and the direct observations of phase state evidentially showed the formation of zinc-CO/CO 2organic complexes in the solid phase of Zn(acac) 2 powder under scCO 2 , which allowed the low temperature synthesis of ZnO particles.These ndings suggest that scCO 2 , with medium solvation power, high diffusivity and small molecular size, has a new potential value to drive the formation of zinc-CO/CO 2organic complexes, which allows the low temperature synthesis of ZnO particles under the dry condition without the use of reactive materials, solvents and post-treatments.

Fig. 2
Fig. 2 (a) XRD pattern and (b) yield of products synthesized at N 2 and CO 2 atmosphere.

Fig. 3
Fig.3XRD pattern of no-washed product synthesized at 30 MPa of CO 2 and referenced materials.

Fig. 4
Fig. 4 (a) TG-MS results and (b) relative intensity of mass spectra at 116 °C for no-washed product synthesized at 30 MPa of CO 2 .(c) Pressure dependence of CO 2 density at 60 °C.(d) Phase state at 30 MPa of CO 2 before and after the reaction.

Fig. 5
Fig. 5 (a) TEM images, (b) TG-MS results, (c) relative intensity of mass spectra at 116 °C and (d) FT-IR spectra for washed product synthesized at 30 MPa of CO 2 .